In most cases, implantable stimulation or stimulating electrodes, for example for heart pacemakers, consist of an electrode body having an insulated cable lead and an electrode head for the transmission of the stimulation pulses, i.e. the active or effective area of the electrode.
Such electrode materials should be compatible with the body, i.e. the formation of connective tissue layers should be very low if it is not suppressed altogether; in any case, the thickness of connective tissue should remain below 100 μm. The stimulation threshold, further, should be as constant as possible. Furthermore, a high double layer capacitance should form at the phase boundary of the electrode and bodily fluid, so that the polarization rise during the stimulation pulses are small, typically less than 0.1V. The high double layer capacitance that is sought has a beneficial effect in the case of stimulation electrodes because, as a result of the impressed current, only slight potential changes occur, electrochemical reactions with the bodily fluid are largely suppressed and the energy outlay is slight.
The demands noted above have been partially met by electrodes in which the electrode head consists of glassy carbon. A high double layer capacitance is achieved by means of an activation of the surface of the glassy carbon, whereby a thin, firmly adhering layer of activated carbon is obtained, i.e. a surface with a microporous structure. Importantly, activated carbon also exhibits good biocompatibility.
However, mechanical processing (during manufacture) and electrical contacting in such a device have proven problematic. This has led to the development of pacing materials based on porous titanium nitride and other nitride or carbide materials which are conducting and have high intrinsic surface areas which meet the capacitance and energy needs of a stimulator.
Published US patent application 2001/0002000 A1 discloses substrates of plastic, metals, etc., with a biocompatible coating, which is formed from amorphous titanium nitride. The applications of the substrates are in the field of cardiac pacemakers and electrodes. European published patent applications EP 0 117 972 A, EP 0 116 280 A and EP 0 115 778 A disclose electrodes for medical applications, provided with porous layers of titanium nitride. U.S. Pat. No. 4,602,637 discloses a cardiac pacemaker system in which the passive electrode is coated, for example, with activated carbon or titanium nitride. German published patent application DE 33 00 672 A1 discloses a cardiac pacemaker system with an electrode which has, for example, a coating of titanium nitride. The publication, J. Riedmüller, A. Bolz, H. Rebling, M. Schaldach, “Improvement of Stimulation and Sensing Performance of Bipolar Pacemaker Leads”, Proceedings of the Annual International Conference of the IEEE/EMBS, pp. 2364-2365 (1992), discloses that the use of titanium nitride layers with anodic polarization leads to the formation of oxide layers in the region of the electrode surface. While the physical properties of the titanium nitride are substantially unchanged in stimulation electrodes with a titanium nitride layer which is used as the cathode, this is not the case with use as an anode. The attack by OH-ions leads to the formation of oxide layers, which cause a rise of impedance and therewith also a rise of the threshold voltage.
U.S. Pat. No. 4,762,136 describes electrodes for use in cardiac pacing having a surface layer of iridium oxide overlying a substrate. U.S. Pat. No. 5,683,443 likewise describes iridium oxide as a non-native coating on a metal electrode surface, and also mentions mixtures of oxides such as ruthenium oxide, iridium oxide and tantalum oxide.
U.S. Pat. No. 5,080,099 describes the use of conductive polymeric hydrogels as a surface layer of an electrode intended to be applied to a patient's skin for exterior stimulation of the heart.
Use of functionalized polypyrrole layers on electrodes and other implantable devices is known for example from WO98/33552 and WO2008/139200. The latter discloses stents having layers which provide good biocompatibility and anti-thrombolytic properties.
WO2008/033546 describes use of polyoxometalates in a surface layer of an implantable electrode, where the layer may be a conductive polymer, and is discussed more below.